If Gmail proved anything, it was that people would, for the most part, accept any terms of service. Or at least not care enough to read the fine-print closely.
gpl does not prevent the owner from changing the licence later. (Unless it is also making use of someone else's gpl components.)
For example, Qt has a free version which is under the GPL; and a paid version which is not. So if you were making software with Qt, if you were using the free version, you'd be compelled to also release your product under GPL. But you could then later switch to a paid subscription and rerelease under some other licience if you wanted to.
Not if the copyright owner changes the license. When you are the creator you can do what you please. With that being said you can not do that if the public writes code. That's why you see CLAs (contributor license agreement)
Important to note that this only applies to future releases by the legal copyright owner. If the community doesn't like it (and they often don't), someone else can fork it from the last time it was GPL, and contributors can abandon the original codebase in favor of the GPL fork. As a result, it is extremely unwise to try to de-GPL software with a lot of contributors, as the copyright holder doesn't have a great chance at competing with a fork if contributors jump ship.
Linus Torvalds could legally pivot Linux to a proprietary license if he wanted to, but we'd probably see it replaced with a fork called "Binix" or something within a few months, and he'd be in charge of abandonware at that point.